A relative few comets are so visually impressive as to be termed "great
comets". Just the right set of circumstances must occur. Far from the
sun, the solid portions of comets, which consist mostly of water ice and
embedded dust particles, are inactive. They are not large enough to be
seen with the naked eye. However, when near the sun, the icy cometary
surfaces vaporize and throw off large quantities of gas and dust thus
forming the enormous atmosphere and tails that make comets so visually
striking. It is the fluorescing of these gases, and particularly the
reflection of sunlight from the minute dust particles in the comet's
atmosphere and tail, that can make these objects so visually impressive.
However, this activity by itself does not insure that a comet will
become a great comet. An active comet can only become great by making a
particularly close approach to the sun so that it produces enormous
quantities of gas and dust or by making a close approach to the Earth so
that its tail can be easily viewed. In either case, great comets must
be seen in a dark sky.

While applying the appellation "great comet" to a particular cometary
return is a subjective process, the following Table is an attempt to
list the great naked-eye comets that have been reported. With the
single exception of periodic comet Halley, all the tabulated comets have
passed through the inner solar system either for the first time or the
intervals between their returns are measured in thousands or millions of
years.

The first tabular entry gives the approximate date when the comet was first
reported as a naked-eye object. The following column gives the approximate
observational interval (in days) during which the comet remained a naked eye
object. The next two columns give the date and distance in
astronomical units
when the comet reached its closest point to the sun (perihelion).
One astronomical unit is approximately the mean distance
between the sun and Earth.
The following columns give the date and distance when the comet
reached its closest point to the Earth (perigee), and the date and apparent magnitude
when the comet reached its brightest in a dark sky.
A diffuse cometary image
becomes noticeable to the naked eye when it reaches a magnitude of
approximately 3.4 in a dark sky.
Compared to a comet whose magnitude is 4,
a 3rd magnitude comet would appear 2.5 times brighter and a magnitude 2
comet would appear 2.5 x 2.5 = 6.3 times brighter still, etc. The brightest
star in the sky (Sirius) has an apparent magnitude of -1.5. At its
brightest, the planet Jupiter appears at magnitude -2.7.

Reported by the Greek historian Ephorus to have split into two
pieces.

The Chinese reported that the tail spanned more than 70 degrees.

The closest approach to the Earth that comet Halley has ever made.
On Apr. 13, the comet's tail was more than 90 degrees in length.

This comet passed very close to the sun and is perhaps the
progenitor of the sungrazing comets of 1882 and 1965 or that of
1843.

Korean observers reported the comet was visible during the daylight
hours on September 9th (probably during twilight only).

On July 26, Chinese observers reported the tail spanning 100
degrees.

Chinese observers reported cloudy weather from October 11 until
Nov. 9, at which time the comet had passed behind the sun.

In mid-March, the comet entered solar conjunction and there were
reports that it was a daylight object for 8 days.

The comet was discovered emerging from solar conjunction.

Last observed on April 20 as it approached solar conjunction.

This was the first comet discovered with the aid of a telescope
(on Nov. 14).

Visible in daylight only 12 degrees from the Sun on February 27.

Tail reported as longer than 90 degrees near Earth close approach.

On the date of perihelion, this sungrazing comet was observed in
daylight nearly one degree from the sun.

Comet observed in southern hemisphere.

The Great September comet was a brilliant object that was observed
very close to the sun, and split into at least four separate pieces
near perihelion. This comet and comet Ikeya-Seki in 1965 are
believed to be members of the same family of sungrazing comets.

This comet was easily observed on January 17 only 4.5 degrees from
the sun. It is often confused with the later apparition of comet
Halley in mid-1910.

On December 18, this comet was seen in daylight only 5 degrees from
the sun. At the end of December, the tail was reported to be
nearly 40 degrees in length.

Sungrazing comet Ikeya-Seki split into two or possibly three pieces
near perihelion. Toward the end of October, the impressive tail
reached lengths in excess of 45 degrees.

The tail of comet Bennett reached 10 degrees in mid-March.

Comet West's impressive broad tail reached a length of 30 degrees on
March 8. Near perihelion, the comet split into four pieces.

The observational interval is based on the time during which the comet
had a total magnitude of 3.4 or brighter.

Naked eye object during January except when too close to the sun
near perihelion and a few days thereafter. Maximum brightness in
mid-January dramatically enhanced due to forward scattering of sunlight
by dust. Impressive dust tail reached 30 degrees and had obvious striae,
similar to comet West in 1976.